A method of manufacturing a bismuth-based superconducting tape wire material will now be described. FIG. 1 shows a manufacturing process of a bismuth high temperature superconducting wire (“Achievement of High Temperature Superconducting Wire Critical Current Exceeding 200 A”, Sumitomo Electric Industries SEI Technical Review, No. 169 published in July 2006). Bismuth-based cuprate powder is charged in a silver pipe, and extended to manufacture a single wire, and a plurality of single wires are collected, and put into a silver pipe for outer sheath, and furthermore extended to be formed into a tape shape in the end. At present, since a silver pipe has a shape with about 100 mm outside diameter, and a tape wire material has about 4 mm width and about 0.2 mm depth (thickness), so it is pressed from all directions to be formed into a tape shape. During this period, necessary calcination is performed. The above-mentioned manufacturing method depends on the point that bismuth-based oxide superconducting material is an anisotropic material, and has superconductive characteristics only in a specified direction. Crystals grow from silver surface (This direction is referred to as “c-axis”), and a perpendicular direction to the c-axis becomes a superconductive direction, and hence a tape shape is used in order to get a large contact area with silver.
FIG. 2 is a cross sectional photograph of a tape wire sample, and FIG. 3 is a magnified view of another tape wire sample. As illustrated in FIGS. 2 and 3, a bismuth-based oxide superconducting material is black-colored, and this portion has a thin extended shape, and referred to as “filament”. It is recognized that while there are many filaments in the center portion, there are less filaments and the cross sectional shape is not a thin tape shape but thick in the end portion. Therefore, in the step that a plurality of thin single wires are put into a silver pipe, many single wires are present at the central portion. Therefore, even if it is rolled to be formed into a tape shape, many superconducting filaments are present in the central portion. The filaments also have thin tape shape. The reason is that crystals grow from the silver surface. By adopting the shape like this (filament shape), it is possible to produce many superconducting filaments, and superconducting characteristics are improved. There are also samples of the same kind of wire material having a circular cross section.
FIG. 4 shows a cross sectional photograph in which the central portion of another sample is further magnified. FIGS. 2 to 4 are photographs in Sumitomo Electric Industries SEI Technical Review (“Achievement of High Temperature Superconducting Wire Critical Current Exceeding 200 A”, NUMBER 169, published in July 2006) similarly to FIG. 1, and an inventor's photograph(s).
In FIG. 4, portions, in which a plurality of filaments are overlapped and silver between the filaments disappears, are seen somewhere. This tendency is widely observed in the present bismuth-based wire. Assuming that the direction in which superconductivity appears is growing from the silver surface, this cross sectional shape is not preferable. If the cross sections of filaments are circular or the cross section area get large at the end portion, the loss, referred to as alternating current loss, gets large, in case of assuming alternating current application.
As shown in FIGS. 2, 3, the region in which filaments are distributed forms an elliptic shape as a whole.
FIG. 5 shows a schematic view of the cross section. The reason is that the cross sectional shape is circular before being rolled to tape shape.
In a structure shown in FIGS. 3, 4, while filaments are often overlapped in the central portion of the tape wire, the filaments are not so frequently overlapped and the shape is also not a thin tape shape in the end portion. The reason is that as the width of the central portion is wide, many silver pipes for segmentation, which will become filaments ultimately, can be packed into a circular cross section of a silver pipe for outer sheath.
Since current is flowed in the superconducting filaments, if the current is flowed at the maximal ability of the tape wire, when taking the current distribution along the longitudinal (x-) direction of the cross section of tape wire material, it results in a distribution having a peak in the central portion. The reason is that many superconducting filaments are present in the central portion of the tape wire material.
FIG. 6 (A) shows one ideal arrangement of filaments. This is the case where filaments are uniformly arranged in the direction of x-axis (abscissa). In this case, a current distribution can be expected to be uniform along the abscissa. If alternating current is flowed in this sample, the current density distribution is as shown in FIG. 6 (B) in many instances. The current density distribution of FIG. 6 (B) is a well-known electromagnetic phenomenon, which generally occurs in a material having a low electric resistivity. This is a phenomenon caused by the fact that an electric field, applied in order to flow current through a tape wire, does not penetrate into the interior part of the conductor immediately, but penetrates from the outer part, and it is called “skin effect”. Since the tape wire material has not only superconducting filaments in the interior part but silver (the electric resistivity is lower than copper), it is a little complicated. And since as for the current density, the current flows so that the inductance is minimal from the principle of least action, the current density is raised at the end portion irrespective of the presence of filaments. As to this kind of analysis, refer to references such as non-patent documents 1, 2 etc.    [Non-Patent Document 1] Svetlomir Stavrev, Bertrand Dutoit, Francesco Grilli, “Self-Field and Geometry Effects in Transport Current Applications of Multifilamentary Bi-2223/Ag Conductor”, IEEE Trans. Appl. Supercond., vol. 13, No. 3, pp. 3807-3813, 2003.    [Non-Patent Document 2] A. C. Campbell, “AC Losses in High Tc Superconductors”, IEEE Trans. Appl. Supercond., vol. 5, No. 2, pp. 682-687, 1995.    [Non-Patent Document 3] Y. Yang, T. Hughes, C. Beduz, D. M. Spiller, R. G. Scurlock, W. T. Norris, “The influence of geometry on self-field AC losses of Ag sheathed PbBi2223 tapes”, Physica C, vol. 256, pp. 378-386, 1996.